Principles of hemodynamics



Blood flow is governed by the basic laws of fluid dynamics. such laws however provide ideal conditions of the moving liquid and the wall of the tubes, conditions that do not occur in the circulatory system, for which the laws of hydrodynamics must be understood only as a theoretical reference system in order to better understand the particular aspects circulation. The circulatory system, to fulfill the important function of transport of the blood, requires energia.Questa is provided primarily from the heart which, contracting, the gives way to the blood. Commonly it is considered that the movement of the blood is determined by a pressure difference; this is only partially true, because other forms of energy influencing the flow of the fluid. The gradient that allows the sliding of the blood is determined by differences in total energy of the fluid that develop between two points of the system. The equation of Bernoulli states that when a fluid flows from one point to another in an ideal system, that is, no acceleration or deceleration, its total energy content remains costante. Nel system circulatory Bernoulli's principle is approximate; In fact, the total energy of the fluid is lost to friction phenomena, ie resistance to the movement of blood from one point to Next, in the form of heat. These energy losses are due to the blood viscosity and to its inertia. Blood viscosity is determined by intermolecular cohesive forces between adjacent layers of blood; it is related to several factors, among which the most important are: the hematocrit, the diameter of the vessels and flow velocity. The inertia is the resistance the blood opposes changes in its state of motion; there total energy loss due to inertia depends on the density of the blood and the square of flow velocity. Since the density of a constant value, you have energy losses due inertia whenever occur sudden changes in speed or direction of flow, as at the level of the branches of the vessels or in pathological situations of stenosis or vascular dilatation. It can be observed as flow and resistance to flow depend, respectively, so directly and inversely proportional to the fourth power of the radius; from these reports it is evident that the modification of the diameter of the vessels is the most effective mechanism important for the regulation of hemodynamics and general district.

The main requirement for the application of the law of Poseuille is laminar flow. Yup defines a laminar flow which flows for coaxial cylindrical layers, in which all particles remove only in parallel to the axis vasal. The individual particles flow with uniform speed in the same cylinder but with different speed among different cylinders, so as to describe a parabolic profile velocity. The blood velocity is maximum at the center of the vessel and virtually nothing on the wall. Above a certain flow regime, the blades are disarrange giving rise to vortices that oriented in all directions: this type of flow is called turbulent. The tendency to turbulence is expressed by the Reynolds number, a dimensionless value that is proportional the ratio between the inertial forces acting on the fluid and the viscous forces. In response to a mechanical stress, all elastic solids, and therefore also the arterial part, is deform, but oppose a force of opposite sign which tends to restore the original shape; the relationship between this force and the surface on which you exercise is defined circumferential stress and is expressed in dynes per square centimeter. In a section of the vessel, represented by a circumference, the circumferential stress is the force that would tend to open the circle if this is interrupted; it depends on the elasticity of the parietal structures, mainly from collagen, and the contractility of muscle cells. Any increase in pressure or radius induces an increase in the circumferential stress; when this exceeds the tensile strength of parietal components (which for the collagen is 5-7 x 107dyne x cm-2) occurs vessel rupture. It is 1900 clear that the imbalance between tension and tensile strength can also occur for the marked reduction of a component parietal, which elastin or collagen. This law allows us to understand why the big aneurysms tend to break more frequently small and because the breakage occurs more often in patients with hypertension than in normotensive ones.


Venous hemodynamics


The venous hemodynamics is more complex than the blood due to several factors, such as collapsibility and distensibility of the vessels, the presence of the valves and muscle pump. veins

have the function of the blood back to the heart arteries that have distributed in the periphery; for the purposes of the circulation, they behave like a tank dynamic, able to quickly change their content, ensuring the return of blood to the heart to adequate cardiac output. the venous system, unlike the arterial blood, is a low pressure system, containing the 75- Approximately 80% of circulating blood volume; This volume can be affected both actively, by modifications of the tone of the wall, as in the case of the passage of the supine to upright, that passively by changes in venous distensibility. In a low pressure system the circumference of the vessel is of elliptical shape and the transition to the circular shape results in a substantial increase in the volume without particular increases in pressure. In the context venous pressure / volume relationship is very characteristic: a small variation pressure are important changes in the circulating blood volume.

In a subject lying, being negligible the effect of the force of gravity, the venous pressur corresponds, at venular, 15 mmHg and 0-5 mmHg in the right atrium. The venous flow is related to the existence of this modest energy gradient and is positively influenced by suction force of the pump respiratory thoracic-abdominal. The flow in the limbs lower assumes a discontinuous trend and phasic with the acts of the breath. During inhalation is observes a decrease in intrathoracic pressure, which leads to a suction of the blood abdomen and veins of the upper limbs and head, while the increase in intra-abdominal pressure for lowering of the diaphragm, which causes a reduction of the outflow of the lower limbs. During exhalation inflows into the chest and slows the rise of the diaphragm results in an increase of the outflow from the lower limbs to the abdomen. When the subject is standing motionless and there is, due to the force of gravity, an increase venous pressure proportional to the distance between the point hydrostatic indifferent, right atrium, and a detection point. The venous pressure at the ankle corresponds to the sum of the value of the hydrostatic pressure, 80-100 mmHg, produced by the weight of the column of blood above, and that of the venular pressure, 15 mmHg.

These conditions are changed when the subject is in motion, due to the pump muscle. In the journey, each contraction of the calf muscles involves a progressive depletion of intramuscular veins, with an acceleration of the flow; this corresponds to a reduction of venous pressure in portions declini.Dopo some muscle contractions, in level of foot venous pressure drops below 30 mmHg. During muscle relaxation the deep circulation and sinusoids are filled with blood capillary and the aspirated from the superficial. The proper functioning of the pump depends by the presence of valves continents and effective functioning of the muscle. several are then the factors that contribute to determining the venous return. They may be schematized in the following way: